1. Chapter 8.7 Acid-Base Titration
Learning Goals: I will be able to…
Calculate the pH during various stages of acid-base titration

2. Acid-Base Titration
Titration is used to determine the concentration of acid/base by adding base/acid of known concentration (titrant).
For most titration, the titrant is slowly added from a burette into a known volume of sample solution in a receiving flask.
The titrant is added just until all the reactant in the sample is consumed (equivalence point).

3. How do you know the equivalence point is reached?
Before beginning an acid-base titration, we mix an appropriate acid-base indicator (substance that changes colour at a certain pH) with the sample.
The endpoint of a titration occurs just as the indicator changes colour.

4. Titrations Involving Strong Acids and Strong Bases

5. Stage 1: Before Titrant is Added

6. Stage 2: After 10.0 mL of Titrant is Added

7. Stage 3: After 50.0 mL of Titrant is Added

8. Stage 4: After 100.0 mL of Titrant is Added The Equivalence Point

9. Stage 5: After 150.0 mL of Titrant is Added

10. pH Curves
The results of these calculations may be illustrated in a graph of pH plotted against volume of titrant added (pH curve)
A strong acid-strong base titration has equivalence point at pH 7

11. Strong acid-Strong Base Titration
Before equivalence point, you may calculate [H+(aq)] (and hence the pH) by dividing the remaining amount of hydrogen ions (in mmol) by the total volume of the solution in the receiving flask (in mL).
At the equivalence point, the pH is always equal to 7.
After the equivalence point, you may calculate [OH-(aq)] by dividing the amount of excess hydroxide ions by the total volume of the solution in the receiving flask. Then obtain [H+(aq)] from Kw, and calculate pH.

12. Strong Base-Strong Acid Titration
Similar to that used in strong acid-strong base titration except that hydroxide ions are in excess before the equivalence point and hydrogen ions are in excess after the equivalence point.

13. Titrations Involving Weak Acids and Strong Bases
To calculate the [H+(aq)], and pH after a certain amount of strong base has been added, we must deal with the weak acid ionization equilibrium.
Even though the acid is weak, it reacts essentially to completion with the hydroxide ion.
At the equivalence point, all of the acid in the flask has reacted.

14. Practice
A mL sample of mol/L ethanoic acid, HC2H3O2(aq) (a weak acid), is titrated with a standardized mol/L solution of sodium hydroxide, NaOH(aq) (a strong base). Predict the amount (in mmol) of unreacted ethanoic acid and the pH of the solution
before titration begins
At the equivalence point when mL of NaOH(aq) has been added
Beyond the equivalence point when a total of mL of NaOH(aq) has been added

15. a) Before titration begins
Step 1: determine the amount of unreacted ethanoic acid molecules in the sample before titration begins using n=cV Step 2: determine the pH of the sample solution using the equilibrium calculation of weak acid. Set up ICE table for dissolution of ethanoic acid Step 3: substitute the equilibrium concentration values into the Ka equation for this equilibrium and solve for x (use the 500 rule if possible): Step 4: use the ICE table and the value of x to determine the hydrogen ion concentration at equilibrium and then calculate pH.

16. b) At the equivalence point when 40.00 mL of NaOH(aq) has been added
Step 1: List the major entities in the sample at the equivalence point. Step 2: determine whether any of the major entities are able to cause hydrolysis. Step 3: write reactions for any hydrolysis reactions and their associated equilibrium constant equations: Step 4: determine the initial concentration of any ions that cause hydrolysis. Step 5: set up an ICE table Step 6: substitute the equilibrium concentration values into the equilibrium constant equation and solve for x (use the 500 rule if possible).

17. c) Beyond the equivalence point when a total of 60
c) Beyond the equivalence point when a total of mL of NaOH(aq) has been added
Step 1: identify the major entities in the solution: Step 2: identify any ions that may cause hydrolysis Step 3: determine the [OH-(aq)], pOH, and pH of the solution

18. pH Curves for Weak Acid-Strong Base Titrations
The equivalence point occurs when the same volume of titrant has been added.
The weaker the acid, the higher the pH value at the equivalence point.

19. Weak Bases-Strong Acids Titrations
The calculations are similar to those of weak acid-strong base titrations.
The major difference is that the strong acid titrant is completely ionized and we have to consider the equilibrium of the weak base sample.
Even though the base is weak, it reacts essentially to completion with the hydrogen ions provided by the strong acid titrant.

20. Weak base-Strong Acid Titrations
Before titration begins, calculate the pH of the sample using the Kb of the weak base.
At the equivalence point, hydrogen ions in the solution are the result of hydrolysis caused by the conjugate acid of the weak base.
Calculate the pH using the Ka of the conjugate acid.
The solution will be acidic: pH < 7
Beyond the equivalence point, determine the pH of the solution by the amount of excess strong acid provided by the titrant.
Determine the pH from the [H+(aq)] produced by the ionization of the strong acid.

21. Acid-Base Indicators
We can make the equivalence point more visible by adding a few drops of an acid-base indicator to the sample.
There are many different acid-base indicators that change colour (have endpoints) at different pH levels.
Ideally, we want the endpoint of a titration to coincide with the equivalence point, thus alerting us that the equivalence point has been reached.

22. Did You Learn?
The progress of a titration is represented by plotting a pH curve showing the pH of the solution against the volume of titrant added.
Strong acid-strong base titrations show a sharp change in pH near the equivalence point.
In strong acid-strong base titrations, pH = 7 at the equivalence point because the pH is determine solely by the autoionization of water:
[H+(aq)] = 1.0 x 10-7 mol/L
In weak acid-strong base titrations, pH > 7 at the equivalence point because of hydrolysis caused by the conjugate base of the weak acid.
In weak base-strong acid titration, pH < 7 at the equivalence point because of hydrolysis caused by the conjugate acid of the weak base.
When calculating pH values for a strong base-weak acid titration, perform stoichiometry first and then equilibrium.
Indicators are often used to mark the equivalence point of an acid-base titration.
An indicator changes colour at its endpoint.
The goal is to have the endpoint and the equivalence point be as close as possible.

23. (remember to supplement your notes!)
HOMEWORK
Required Reading:
p. 540 – 557
(remember to supplement your notes!)
Questions:
P. 547 #1, 2
P. 554 #1, 2
P. 557 #1-8